Basic Countermeasures Equipment
1080TDR - TIME DOMAIN REFLECTOMETER (TDR) PULSER
The 1080TDR is designed for use with any oscilloscope with a bandwidth in excess of 20MHz. It provides the necessary pulses to drive any wire pair to determine line mismatches. The unit has sufficient output to drive over two miles of wire. Powered by two standard 9-volt batteries, the 1080TDR measures 1x3x5 inches. Supplied with three input cables and scope output cable (BNC).
The 1080TDR, when used with an external oscilloscope, analyzes wire pairs or transmission lines. The TDR is essentially an echo ranging device. It generates a short, very fast rise time pulse that travels along the wire pair at a speed determined by the velocity factor of the wire. When a discontinuity is encountered, the pulse is reflected back along the wire pair to the 1080TDR and oscilloscope. The actual distance to this discontinuity can be measured by simple calculation (keep a calculator handy).
OPERATION: Connect the 1080TDR to the vertical input of the oscilloscope. Set the vertical sensitivity to .5 volts per division and the horizontal sweep at .5 microseconds (uSec) per division. Three different pulse lengths are generated; .1 uses (SHORT) for lines up to 2,500 feet, .2 uSec (MEDIUM) for lines up to 10,000 feet and .5 uSec (LONG) for lines up to 20,000 feet. Set the TDR on LONG pulse. Sync the scope. Set the horizontal position so the leading edge of the pulse is at the left-hand edge of the calibration marks on the screen. The rise time of the pulse is very fast and difficult to see. Increase the intensity momentarily if required. Set the vertical position so the trace rides on the center line of the screen.
Connect a known length of telephone line or zip cord to the RED terminals (100 feet will do). With the TDR PULSE switch on S and the scope horizontal sweep set at .1uSec per division and the far end of the test line open circuited the following positive going pulse will appear.
If the far end is short circuited the following negative going pulse will appear.
Measure the distance in microseconds to where the reflected pulse BEGINS TO LEAVE THE BASE LINE of the sweep. For 100 feet of wire with a velocity factor of .67 the pulse should occur at roughly .3uSec. Use the formula
Therefore, 1000 X .67 X .3 DIVIDED BY 2 = 100 FEET. The divide by two is necessary because it compensates for the fact that the pulse has to travel to the discontinuity and reflect back.
When making TDR measurements bear in mind that the length of the pulse itself can mask the very near measurements. The .1uSec pulse will mask the first 35 feet of cable, the .2uSec pulse will mask the first 70 feet and the .5uSec pulse 170 feet (this is called a dead zone). This does not mean that measurements can not be made at lengths below 35 feet. To test short cables use the LONG pulse and make measurements on the TOP OF THE PULSE PEDESTAL itself. When doing this it will be necessary to adjust the vertical position so the top of the pulse is on the center line.
When testing long lines it may be necessary to advance the vertical sensitivity to compensate for the loss of signal down-line.
Here are a few signatures of the return signal...
SHORTED CONDUCTOR The reflection from a shorted pair is a negative or downward pulse. There will be no reflection from the far end of the cable.
SPLICES A splice produces a positive pulse followed immediately by a small negative overshoot.
CAPACITOR BUILD-OUT NETWORK A network presents a low shunt impedance to the pulse and produces a negative cr downward reflection followed by a small positive overshoot.
LOAD COIL A load coil presents a high series impedance to the pulse and a positive or upward reflection. It is not possible to see beyond the first load coil.
CROSSED OR SPLIT PAIRS The splice at which the split occurs is indicated by a negative reflection. The resplit is indicated by a pulse of opposite polarity, but of smaller amplitude. First locate and clear the split, then the resplit.
BRIDGE TAP The tap appears as a negative pulse followed immediately by a slight positive overshoot. Because of multiple reflections in this test, it is difficult to trace circuits that contain several taps. Test progessively by moving from tap to tap.
Helpful hint. If the line under test is terminated by a telephone set or linemans handset place an audible beeper by the telephone handset or linemans handset. The reflected pulse as seen on the oscilloscope will pulse up and down at the rate of the beeper.
Formulas: TIME always in uSeconds. Vf means Velocity Factor
Unknown Length (L) = 1000 x VF x TIME
Unknown Velocity (VF) = (2 X LENGTH)/1000 x TIME
Unknown TIME = (2 X LENGTH)/1000 x VF
The 1080TDR has an output impedance of less that 50 ohms.
Initial reflected pulses arriving back at the 1080TDR/Oscilloscope will again be reflected, travel back down the line to be reflected by the fault thus causing a second or "ghost" reflection at twice the fault distance. Watch for ghost reflections.
Multiple faults may or may not be observed. If the first fault encountered is severe, most or all of the pulse energy will be reflected thereby disguising other possible faults. After clearing the first indicated fault, go on to the second.
Telephone cables by nature of their construction produce displays that are not "clean", i.e. free from bumps.
Velocity Factors (VF) can vary from .5 to .8. If the VF is unknown, start with .67.
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